Windows are provided in buildings in order to allow sight outwards and inwards and for admitting daylight to illuminate spaces within the buildings. Deep spaces in buildings typically obtain day-lighting in a zone close to the windows whereas zones far from the windows may require artificial lighting to reach a sufficient illumination level. In essence, a bright light near to the windows make the eyes adapt in such a way that regions far from the windows are experienced as too dark, even in the presence of additional lighting. The differences in illumination in a building space caused by day-lighting levels can be appreciated by realizing that full sunlight outdoors may be of the order of 100 klx (kilolux=1000 candela/m2), while a TV-studio typically is lit to 1 klx and an artificially well-lit office has 0.4 klx. Illumination in areas far from a window that is considered to be sufficient when it is dark outside, can be experienced as totally insufficient when clear sun light is present at the window. The reason is that the eye adapts its sensitivity to the highest of the different levels of illumination within sight. In practice the eye adapts to regions close to the window when the sun light is present, which makes the areas far from the window to appear as very dark, even if the absolute illumination level is appropriate. Furthermore, very high contrasts tend to appear as mirror images at e.g. computer screens or other glossy surfaces.
One approach to overcome this situation is to use some kind of arrangement at the window for keeping the strong light out. This could e.g. be a pure mechanical arrangement, such as a curtain, or more technically advanced systems, such as electrochromic windows e.g. disclosed in “Application issues for large-area electrochromic windows in commercial buildings” by E. S. Lee and D. L. DiBartolomeo in Solar Energy Materials & Solar Cells 71 (2002) pp. 465-491. However, the indoor space will then typically be so dark that artificial light is necessary even in day time, which increases the need for electrical power used for illumination purposes.
A problem with conventional indoor illumination is thus that day light entering windows cause too high illumination in areas close to the window, which prohibits the eye to adapt to the illumination level at areas far from the window. Solutions according to prior art of this problem in turn lead to another problem of increased power consumption for illumination purposes.